I suppose you'd be buying some kind of generated token that doesn't correspond to your account ID in any way, then doing some fancy math to prove to the Signal servers that you have such a token without revealing which one it is, while at the same time preventing the same token being used more than once. Given that there's no need to tie it to any real-world attributes of the user (i.e. this is not like "age verification") it seems like this should be possible, but I'm curious about the details.